Conventional trading call systems are built by accommodating multiple trading call terminals in a line control unit via an ISDN basic interface (2B+D) circuit. Each trading call terminal in the system transmits and receives environmental information, such as which line received a message or which line is being used by another trading call terminal and under what circumstance, using a D channel of the ISDN basic interface. The status can be monitored by the indication on the line display lamp of a trading call terminal. Based on these functions, actions can be taken, such as making a judgement as to whether to accept or reject a receiving call knowing the receiving line (the calling party) or avoiding a call being made to a line being used.
In the meantime, a trading call terminal is required to acquire multiple lines simultaneously and to communicate to each of them, and further, to transmit and receive environmental information therefor to and from the line control equipment. When a trading call terminal is connected to the line control unit via a commercial switched circuit, environmental information cannot be transmitted or received, as the voice information alone occupies multiple lines, and therefore, the use of a trading call terminal under the same environment as the trading room was difficult.
Furthermore, even in the case of using an ISDN circuit as the commercial switched circuit, environmental information cannot be transmitted as is, as the voice information occupies two B channels and call control data occupies the D channel.
On the other hand, as trading transactions deal with both the domestic market and the overseas market, the trader has to stand by in the trading room to adjust to the customer's local time if he is dealing with a customer in a country having a different time zone. In order to cope with such situation, the demand is growing for an environment where a call can be made similarly as in the trading room by installing a trading call terminal at home.